This invention relates to high-temperature, secondary electrochemical cells and batteries of such cells that can be employed as power sources for electrical automobiles, hybrid electric vehicles or for the storage of energy generated during intervals of off-peak power consumption. It is particularly applicable to electrochemical cells that employ metal sulfides as positive electrode reactants and lithium-aluminum alloy as he negative electrode reactant.
A substantial amount of work has been done in the development of these types of electrochemical cells and their electrodes. Various type cells showing promise have employed lithium, lithium-aluminum alloy or sodium as the reactant within the negative electrode. In the positive electrode, the chalcogens, particularly sulfur and sulfur compounds, have been used. Electrolytes of molten salt generally containing the ions of negative electrode reactant are used to provide ionic conduction between the electrodes. Examples of such secondary cells and their various components are disclosed in U.S. Pat. No. 3,716,409 to Cairns et al., entitled "Cathodes for Secondary Electrochemical Power-Producing Cells," Feb. 13, 1973; U.S. Pat. No. 3,666,560 to Cairns et al., entitled "Electrochemical Power-Producing Cell,"May 30, 1972; and U.S. Pat. No. 3,488,221 to Hiroshi Shimotake et al., Jan. 6, 1970. Other cells and electrodes of these types are illustrated in U.S. Pat. No. 3,833,421 to Rubischko et al., entitled "Secondary Electrochemical Cells with a Chalcogen Cathode," Sept. 3, 1974, and U.S. Pat. No. 3,827,910 to Cairns et al., entitled "Homogeneous Cathode Mixture for Secondary Electrochemical Power-Producing Cells,"Aug. 6, 1974. Other pending patent applications showing electrochemical cells more closely related to the type described in the present application include patent application Ser. No. 416,311, entitled "Modular Electrochemical Cell," filed Nov. 15, 1973, to Walsh et al., now U.S. Pat. No. 3,887,396 and Ser. No. 434,459, entitled "Cathode for a Secondary Electrochemical Cell," to Gay et al., filed Jan. 18, 1974. Each of these patents and patent applications are assigned to the assignee of the present application.
As is discussed in the pending patent application to Gay, cited above, reductions in sulfur activity and loss can be made through use of metal sulfides as the cathode material. For instance, electrochemical cells have been proposed which employ lithium or lithium-aluminum alloy as the anode reactant along with metal sulfide such as FeS.sub.2, FeS, CoS.sub.2, Co.sub.3 S.sub.4, NiS.sub.2, MoS.sub.3 or Cu.sub.2 S as the positive electrode reactant. (Etude Thermodynamique des Generateurs a Electrode de Lithium, Entropie, No. 4, pp. 24-34, Juillet-Aout 1971, Caiola et al.) Other cathode reactants that have been considered include Sb.sub.2 S.sub.3, As.sub.2 S.sub.2, As.sub.2 S.sub.3 and P.sub.4 S.sub.10. Typical reactions with the above materials within a cell having a lithium or a lithium alloy anode are as follows: EQU 4e.sup.- + FeS.sub.2 + 4Li.sup.+ .fwdarw. 2Li.sub.2 S + Fe EQU 2e.sup.- + FeS + 2Li.sup.+ .fwdarw. Li.sub.2 S + Fe EQU 4e.sup.- + CoS.sub.2 + 4Li.sup.+ .fwdarw. 2Li.sub.2 S + Co EQU 8e.sup.- + Co.sub.3 S.sub.4 + 8Li.sup.+ .fwdarw. 4Li.sub.2 S + 3Co EQU 2e.sup.- + NiS + 2Li.sup.+ .fwdarw. Li.sub.2 S + Ni EQU 6e.sup.- +MoS.sub.3 + 6Li.sup.+ .fwdarw. 3Li.sub.2 S + Mo EQU 2e.sup.- + Cu.sub.2 S + 2Li.sup.+ .fwdarw. Li.sub.2 S + 2Cu.
Correspondingly, the reaction at the negative electrode is: EQU Li .fwdarw. Li.sup.+ +e.sup.- or EQU LiAl.fwdarw. Li.sup.+ + Al + e.sup.-.
In preparing electrochemical cells of this type, various problems have been encountered. Lithium metal is most reactive and is easily contaminated by combination with moisture, oxygen or nitrogen within air. Li-Al alloys employed as negative electrodes are often in the form of a porous compact or plaque with high surface area, thus increasing their reactivity. Consequently, lithium and lithium alloys are ordinarily handled in a dry and inert gas such as in a helium environment in the preparation of electrodes and in the assembly of electrochemical cells. Quite frequently, glove box type facilities are employed to provide this inert atmosphere and to insure the safety of employees.
Other difficulties have arisen as a result of swelling and distortion, particularly within the positive electrode. These distortions have caused current leakage and electrical shorts within the cell. Where iron sulfides are employed as the positive electrode reactant, it combines with lithium ions to form Li.sub.2 S and iron metal with a volume increase of about 2.6 to 1 in the positive electrode. Sufficient void space will ordinarily be provided within the positive electrode to accommodate an expansion of this magnitude. However, experience in testing high-temperature cells of these types has shown nonuniform expansion of the positive electrode, possibly due to nonuniform current flow. The resulting distortion will produce electrical shorting even in cells having adequate space for the predicted expansion. In other instances, electrically conductive grids or mesh employed as current collectors have been distorted and interrupted, resulting in reduced effectiveness.
Therefore, in view of these problems associated with prior electrochemical cells, it is an object of the present invention to provide an electrochemical cell that can be easily assembled with a reduced likelihood of contamination.
It is also an object to provide an electrochemical cell with a positive electrode having a reduced risk of swelling or distortion.
It is a further object to provide an electrochemical cell with an improved uniformity in the distribution of reaction products within the positive electrode.